Synthesis, structure and surface properties of some mesoporous cero-phosphoro-aluminates

Abstract

Mesoporous cero-phosphoro-aluminates (Al₁₀₀P_XCe_Y, X, Y = 0, 5, 10, 20) have been prepared and characterized by XRD analysis, surface area and porosity measurements, XPS analysis and surface acidity determination. The preparation of the Al₁₀₀P_XCe_Y solids took place in an aqueous environment with NH₃ as precipitating agent and heating at 600 °C. The addition of phosphorus leads to an increase in the specific surface area (SSA) of the γ-Al₂O₃ parent solid from 230 to 390 m² g⁻¹ and the formation of amorphous aluminophosphates. The addition of cerium does not result in its incorporation into the alumina or the aluminophosphate solids but a separate phase of CeO₂ is formed at X,Y < 20 while at X,Y = 20 CePO₄ becomes apparent. At the same time the specific surface area drops from 230–390 m² g⁻¹ to 170–180 m² g⁻¹ depending on the sample. All the Al₁₀₀P_XCe_Y samples are truly mesoporous with mean pore diameters around 6–10 nm and full width at half maximum (FWHM) of the pore size distribution at 3–7 nm. The size of the crystallites of CeO₂ (Sherrer relationship) is around 4–8 nm depending on the sample. The surface acid density, determined by NH₃ adsorption, increases linearly with the addition of phosphates from 5 × 10¹⁷ sites m⁻² for X = 0 to 13 × 10¹⁷ sites m⁻² for X = 20, while it is practically invariant on addition of cerium. The XPS analysis showed that the surface of those porous materials is highly enriched in cerium as compared to the nominal solid composition. The enrichment is more substantial in pure alumina, reaching relative values of 100–150%, and decreases with the addition of phosphorus, where the relative enrichment is 30–60%. The surface of the solids without or with the addition of a small amount of cerium (Y = 0,5,10) is moderately enriched in phosphorus (10–30%) while for high cerium content (Y = 20) the surface is moderately depleted of phosphorus to 14–23%. The surface enrichment, or depletion, in phosphorus and/or cerium is corroborated by linear relationships between the surface composition (XPS) and the acidity (TPD NH₃) in the case of phosphorus or the intensity of the main reflection in XRD of CeO₂ in the case of cerium.